Field of the Invention
The invention relates to a method for operating a magnetic-inductive flowmeter for measuring the flow velocity of a flowing medium. The magnetic-inductive flowmeter has a magnetic field generator with an electric magnet arrangement and a permanent magnet arrangement for generating a magnetic field with varyingly large magnetic flux densities in the flowing medium. Furthermore, the magnetic-inductive flowmeter has a measuring electrode arrangement for measuring the measuring voltage induced by the magnetic field in the flowing medium and superimposed by an interfering voltage.
Description of Related Art
Magnetic-inductive flowmeters have been known extensively for decades in the prior art. As an example, reference is made to the citation “Technische Durchflussmessung” [Technical Flow Measurement] by Prof. Dr.-Ing. H. W. Bonfig, 3rd edition, Vulkan-Verlag, Essen, 2002, pages 123 to 167, and also to the citation “Grundlagen Magnetisch-Induktive Durchflussmessung” [Fundamentals of Magnetic-Inductive Flow Measurement] by Dipl.-Ing. Friedrich Hofmann, 3rd edition, 2003, publication of the company KROHNE Messtechnik GmbH & Co. KG.
The basic principle of magnetic-inductive flowmeters goes back to Michael Faraday, who, in the year 1832, suggested using the principle of electromagnetic induction for measuring the flow velocity of an electrically conductive medium. According to Faraday's law of induction, an electric field is generated that is perpendicular to the direction of flow of the medium and also perpendicular to the magnetic field in a flowing, electrically conductive medium interfused by a magnetic field.
Faraday's law of induction is made use of in magnetic-inductive flowmeters of the type described in the introduction, in that the magnetic field generator provides a magnetic field, which interfuses the flowing medium. The electric magnet arrangement has at least one electric magnet and the permanent magnetic arrangement has at least one permanent magnet. The magnetic field interfusing the medium is generated by the electric magnet or electric magnets with current flowing through it/them and by the permanent magnet or permanent magnets, wherein the individual magnetic fields of the electric magnet or magnets with current flowing through it/them and of the permanent magnet or magnets overlap one another. Varyingly large magnetic flux densities in the flowing medium are applied with different currents being fed from at least one of the electric magnets. The currents can, thereby, differ from one another in algebraic sign as well as in absolute value.
The magnetic field in the medium has at least one component perpendicular to the direction of flow of the medium, whereby an electric field is formed in the medium perpendicular to both the direction of the flowing medium as well as to the direction of the magnetic field. The electric field strength of the electric field is a measure for the flow of the medium through the magnetic-inductive flowmeter.
The measuring electrode arrangement has at least two measuring electrodes, which are in galvanic contact with the medium flowing through the magnetic-inductive flowmeter. The arrangement of at least two of the measuring electrodes is preferably opposite one another on a common axis parallel to the direction of the electric field with a large as possible distance from one another, whereby the measuring voltage caused by the electric field between the measuring electrodes is at a maximum. The measuring voltage is a measure for the electric field strength and the measuring device is designed for determining the flow using the measuring voltage. Measuring electrode arrangements are also known, in which the measuring electrodes capacitively tap the measuring voltage.
Magnetic-inductive flowmeters having an electric magnet arrangement and no permanent magnet arrangement for generating a magnetic field are mostly operated with alternating magnetic fields in flowing medium. An alternating magnetic field creates an alternating measuring voltage, whereby, at least in part, a compensation of the interfering voltage is possible. A requirement for a compensation of the interfering voltage is that the temporal change of the interfering voltage is slower than the temporal change of the alternating magnetic field. Additionally, the electrochemical interfering voltage contributes to the interfering voltage.
The alternating magnetic field can be a harmonic alternating magnetic field. The temporal change of the magnetic field strength is a harmonic oscillation in harmonic alternating magnetic fields. A harmonic alternating magnetic field can be generated by feeding the electric magnet arrangement from an available alternating voltage net. The operation of magnetic-inductive flowmeters with a harmonic alternating magnetic field, in turn, has disadvantages as can be seen in DE 199 07 864 A1, column 1, line 53 to column 2, line 13, corresponding parts of U.S. Pat. No. 6,453,754 B1.
Disadvantages that result during operation of a magnetic-inductive flowmeter having a harmonic alternating magnetic field can be avoided using an alternating magnetic field that is a switched constant magnetic field. A switched constant magnetic field consists of a periodically repeating sequence of at least two intervals, wherein in each of the intervals, the magnetic field is constant after a transient period and the magnetic fields are different in two consecutive intervals. Different magnetic fields are generated by applying current with different current values to at least one electric magnet. A current is, thereby, characterized by the absolute value of the current and the direction of the current. Thus, magnetic fields can be differentiated using different magnetic field strengths and/or different magnetic field directions. For the most part, a switched constant magnetic field consists of two intervals of the same length and the magnetic fields of the intervals in the steady state have magnetic field strengths with the same absolute value, but opposing magnetic field directions.
In magnetic-inductive flowmeters of the type described in the introduction and known from the prior art, the electric magnet arrangement is used only for changing the magnetic remanence of the permanent magnet arrangement, whereby the power consumption of the magnetic-inductive flowmeter is to be decreased.
The energy requirements of the electric magnet arrangement for changing the magnetic remanence of the permanent magnet arrangement, however, is much higher in comparison to the generation of the magnetic field interfusing the flowing medium using only the electric magnet arrangement. For this reason, the frequency of the change of the magnetic remanence of the permanent magnet arrangement is less than the frequency of the alternating magnetic field in magnetic-inductive flowmeters with only an electric magnet arrangement. The low frequency of change of the magnetic remanence, however, also results in worse compensation of the interfering voltage.